1. Field of the Invention
The present invention relates to an on-press development-type lithographic printing plate precursor on which an image can be recorded by scanning an infrared laser ray based on digital signals of a computer or the like.
2. Background Art
The lithographic printing plate in general consists of a lipophilic image area of receiving an ink in the printing process and a hydrophilic non-image area of receiving a fountain solution. The lithographic printing is a printing method utilizing the repellency between water and an oily ink from each other, where the lipophilic image area of the lithographic printing plate and the hydrophilic non-image area are formed as an ink-receiving part and a fountain solution-receiving part (ink non-receiving part), respectively, to cause difference in the ink adhesion on the surface of the lithographic printing plate, an ink is attached only to the image area and thereafter, the ink is transferred to a material on which the image is printed, such as paper, thereby performing printing.
For producing this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (image recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method where the lithographic printing plate precursor is exposed through an original image such as lith film and while leaving the image recording layer in the portion working out to the image area, the other unnecessary image recording layer is dissolved and removed with an alkaline developer or an organic solvent to reveal the hydrophilic support surface, thereby forming a non-image area.
In the plate-making process using a conventional lithographic printing plate precursor, a step of dissolving and removing the unnecessary image recording layer with a developer or the like must be provided after exposure but as one problem to be solved, it is demanded to dispense with or simplify such an additive wet processing. In particular, the treatment of a waste solution discharged accompanying the wet processing is recently a great concern to the entire industry in view of consideration for global environment and the demand for solving the above-described problem is becoming stronger.
As one of simple plate-making methods to cope with such a requirement, a method called on-press development has been proposed, where an image recording layer allowing for removal of the unnecessary portion of the image recording layer in a normal printing process is used and after exposure, the unnecessary portion of the image recording layer is removed on a printing press to obtain a lithographic printing plate.
Specific examples of the on-press development method include a method using a lithographic printing plate precursor having an image recording layer dissolvable or dispersible in a fountain solution, an ink solvent or an emulsified product of fountain solution and ink, a method of mechanically removing the image recording layer by the contact with rollers or a blanket cylinder of a printing press, and a method of weakening the cohesion of the image recording layer or adhesion between the image recording layer and the support by the impregnation of a fountain solution, an ink solvent or the like and then mechanically removing the image recording layer by the contact with rollers or a blanket cylinder.
In the present invention, unless otherwise indicated, the “development processing step” indicates a step where, by using an apparatus (usually an automatic developing machine) except for a printing press, the image recording layer in the portion unexposed with an infrared laser of a printing plate precursor is removed through contact with a liquid (usually an alkaline developer) to reveal the hydrophilic support surface, and the “on-press development” indicates a method or step where, by using a printing press, the image recording layer in the portion unexposed with an infrared laser is removed through contact with a liquid (usually a printing ink and/or a fountain solution) to reveal the hydrophilic support surface.
However, when an image recording layer for conventional image recording systems utilizing ultraviolet ray or visible light is used, the image recording layer is not fixed after exposure and therefore, for example, a cumbersome method of storing the exposed lithographic printing plate precursor in a completely light-shielded state or under constant temperature conditions until loading on a printing press must be taken.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-output systems coping with such a digitization technique have been put into practical use. Along with this, a computer-to-plate technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate without intervention of a lith film. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
As described above, the demand for a simplified, dry-system or non-processing plate-making work is ever-stronger in recent years from both aspects of consideration for global environment and adaptation for digitization.
In recent years, a high output laser such as semiconductor laser and YAG laser is inexpensively available and a method using such a high output laser for the image recording means is promising as a method for producing a lithographic printing plate by scanning exposure which is readily incorporated in the digitization technique.
In a conventional plate-making method, imagewise exposure of low intensity to medium intensity is applied to a photosensitive lithographic printing plate precursor, and the image recording is effected by utilizing an imagewise change in the physical properties resulting from a photochemical reaction in the image recording layer. On the other hand, in the method using a high output laser, a large quantity of light energy is irradiated on the exposure region for a very short time to efficiently convert the light energy to heat energy and by the effect of this heat, a chemical change, a phase change or a thermal change such as change of morphology or structure is caused and utilized for the image recording. Accordingly, image information is input by light energy such as laser light, but image recording is performed by a reaction due to heat energy in addition to light energy. The recording system making use of heat generation by such high power density exposure is usually called heat-mode recording and the conversion from light energy to heat energy is called light-to-heat conversion.
A great advantage of the plate-making method using heat-mode recording is that the image recording layer is not sensitized by light of normal intensity level such as room lighting, and fixing of the image recorded by high intensity exposure is not indispensable. That is, the lithographic printing plate precursor used for heat-mode recording is free from fear of being sensitized by room light before exposure and not required to fix the image after exposure. Accordingly, for example, when an image recording layer which is insolubilized or solubilized by exposure with a high output laser is used and a plate-making process of imagewise processing the exposed image recording layer to produce a lithographic printing plate is performed by on-press development, a system where even if the printing plate precursor is exposed to environmental light in a room after exposure, this does not affect the image, can be established. In this way, it is expected that when heat-mode recording is utilized, a lithographic printing plate precursor suitable for on-press development can be obtained.
A laser is recently making a remarkable progress and particularly, as for the semiconductor laser and solid laser of emitting an infrared ray at a wavelength of 760 to 1,200 nm, a high-output and compact laser becomes easily available. Such an infrared laser is very useful as a recording light source at the direct production of a printing plate from digital data of a computer or the like.
However, many photosensitive recording materials useful in practice as the image recording layer have sensitivity in the visible light region at a wavelength of 760 nm or less and therefore, image recording cannot be performed by an infrared laser. A material allowing for image recording by an infrared laser is demanded.
In this connection, for example, Patent Document 1: Japanese Patent No. 2,938,397 describes a lithographic printing plate precursor where an image-forming layer comprising a hydrophilic binder having dispersed therein hydrophobic thermoplastic polymer particles is provided on a hydrophilic support. In Patent Document 1, it is stated that after exposing this lithographic printing plate precursor by an infrared laser to cause coalescence of hydrophobic thermoplastic polymer particles by the effect of heat and thereby form an image, the lithographic printing plate precursor can be loaded on a cylinder of a printing press and on-press developed with a fountain solution and/or an ink.
Such a method of forming an image through coalescence by mere heat fusion of fine particles has a problem that despite good on-press developability, the image strength (adhesion to the support) is extremely low and the press life is not satisfied.
Patent Documents 2 and 3: JP-A-2001-277740 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2001-277742 describe a lithographic printing plate precursor comprising a hydrophilic support having thereon a layer containing a polymerizable compound-enclosing microcapsule.
Also, Patent Document 4: JP-A-2002-287334 describes a lithographic printing plate precursor comprising a support having provided thereon a photosensitive layer containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound.
The method using a polymerization reaction is characterized in that as compared with the image area formed by heat fusion of polymer fine particles, the image strength is relatively good by virtue of high chemical bonding density in the image area. However, in the practical viewpoint, the on-press developability, the press life and the polymerization efficiency (sensitivity) all are not yet satisfied and such a method is not used in practice.